Updating streamcorpus.thrift with several small changes and one
significant change:

1) significant: Token.mention_id is now available for differentiating
adjacent multi-token mentions that are in the same coref chain but
actually different mentions.

2) token_num can now be up to 2**31 instead of 2**15, which might be
needed for long docs

3) fixed several comments

4) Token.token and Token.lemma should be a string, because Tokens are
generated from clean_visible, which is always supposed to be valid
unicode, so we can reliably convert it to UTF-8

5) made TaggerID a separate typedef for clarity, no structural change

6) several comments improvements

7) set version to v0_2_0 because 0_1_0 should correspond to kba.thrift
from TREC KBA 2012

8) dangling question about whether target_id should be a full URL;
need to answer this.